THE LOESS OF NATCHEZ MISS. 319 



2. The agency. — The objections to water and ice as agen- 

 cies of transportation have already been briefly stated. Wind 

 does carry large quantities of material, and in the loess regions 

 winds are strong and frequent during the summer season when 

 the soils are loose and easily eroded. Moreover, the general 

 southerly course of the larger streams in the Mississippi val- 

 ley, along which most of the loess is deposited, favors the con- 

 centration of the prevailing southerly winds in the troughs of 

 the valleys, through which they crowd with increased force, 

 and dislodge the fine particles of detritus brought down by 

 the streams and exposed in bars. In the north drifting snows, 

 especially in the Missouri river region, also gather up and 

 carry great quantities of dust. 



3. — J lie anchorage. — The material so gathered up must be 

 deposited in a place from which it cannot easily be dislodged 

 by erosion if it is to add to the sum-total of the deposit. It 

 is a well known fact that plants check or prevent erosion. 

 The more luxuriant the vegetation the less erosion is pro- 

 duced by rainstorms. The most violent rainstorms will 

 scarcely disturb the finest leaf-mould even on very steep 

 slopes in the woods, while they wash out great quantities of 

 material in more open country. Vegetation, and especially 

 forest vegetation, is best developed along the streams. 



It will thus be seen, that these three favorable conditions, 

 while not restricted to streams, are yet best developed adjacent 

 to them, and could accomplish more than would be possible 

 at more remote points. That this is true appears also from 

 the evidence of the snail fauna. The loess is thickest, and 

 also most fossiliferous, in close proximity to streams, and near 

 them, too, modern land-snails are most abundant. The fact 

 that fossils are more abundant near streams than they are in 

 more remote regions, does not indicate a difference in the or- 

 igin of the respective deposits,* but merely further shows that 

 torrestrial conditions along streams, even on high grounds 



*See Hershey's article in Am. Geol. , vol. XXV, pp. 369-374. — L900. 



